This invention relates to an apparatus for feeding and metering fluid components to a mixing head, in particular for feeding and metering reactive chemical components such as polyhidric alcohol and isocyanate or others, to a high pressure mixing head.
There are generally-known apparatuses for metering and mixing two reactive chemical components, comprising a component metering and feeding assembly, in which the metering and feeding unit of each component consists of a hydraulically-actuated piston pump, connected to the storage tank of the component and, respectively, to a mixing head. In particular, the German patent DE-C-33 09 964 refers to an apparatus of the above-mentioned kind, in which the hydraulic circuits controlling the component feeding and metering units are connected to a delivery pump common to both circuits, of the type controlled in such a way that each of the hydraulic circuits for feeding the control fluid is provided with a flow control valve which can be controlled according to the speed of the piston assembly of each unit, and in which the hydraulic circuits are further connected to the control inlet of the fluid delivery pump.
It is also desirable to feed the chemical components into the chamber of a mixing head under controlled conditions with regard to pressure, temperature and quantity, so that the mixing takes place in the desired stoichiometric ratios and within the established reaction times, in order to prevent the mixture from solidifying before reaching the mould. In particular, in high pressure mixing heads, it is essential to have a precise adjustment of the component feeding pressure, which must be maintained at a pre-established constant value, due to the fact that this pressure in substance constitutes the energy that gives rise to the correct mixing of the two jets of components which impinge at high speed in the mixing chamber. At present, the pressure is adjusted manually at the beginning of each stage of the work, or at the startup of the installation, by acting on the injection nozzles of the mixing heads, without any possibility whatsoever of doing so while the apparatus is working in that it would involve stopping or interrupting the work.
The above constitutes a limit in the development of the present apparatus in that there are causes of disturbance or alteration of the mixing conditions, which can take place when the apparatus is working, about which nothing can be done immediately. The use of automatic control systems in addition to the already existing ones is rather unadvisable in the present apparatuses, in that it could cause disturbance or interference with the other existing control equipment, with the risk of causing the entire control system to resonate.
The problem of keeping the pressure and flow rate of the components constant in high pressure mixing heads is consequently of crucial importance. In the known apparatuses, it has therefore been suggested to control the constancy of the flow rates of the components, in relation to their feeding speed, and to vary the flow rate of the hydraulic control fluid in relation to variations in the aforesaid feed speed, whenever greater power is required in the metering units in order to keep the flow rates of both components constant. For this reason, with the known apparatuses, it is not possible by means of adjustment to affect the causes which tend to alter the pressure required for feeding and injecting the components into the chamber of a mixing head; causes which for the most part can be attributed to residual substances or solidified particles of components in the nozzles of the injectors which consequently give rise to variations in the component injection pressure and in the mixing conditions.
A scope of this invention is to provide an apparatus for metering and feeding reactive chemical components into high pressure mixing heads, which makes use of hydraulically-controlled piston-type pumping and metering units, by which it is possible to remedy the abovementioned problems and to ensure that the components are fed into the mixing chamber at a constant pressure and rate of flow.
A further scope of this invention is to provide an apparatus as described above, which is extremely simple in structure, capable of operating with a single pump for the hydraulic control circuit and at the same time utilizes a single automatic control device, for each individual component, which is capable of ensuring constant pressure and flow rate values.
A still further scope of this invention is to provide an apparatus as described above, in which the hydraulic control circuit can operate with comparatively lower pressure values, due to the fact that the causes of loss of hydraulic power have been eliminated or reduced to a minimum.